/Engine/Source/Runtime/MovieScene/Public/Containers/SparseBitSet.h
The hash allows for empty buckets to be completely omitted from memory, and affords very fast comparison for buckets that have no set bits. This container is specialized for relatively large bitfields that have small numbers of set bits (ie, needles in haystacks) as they will provide the best memory vs CPU tradeoffs.
template<typename HashType, typename BucketStorage> struct TSparseBitSet